Oil-immersed self-cooled transformers are widely used in distribution systems; however, the complex structure and large number of fins in their plate-type radiators often pose challenges to thermal modeling and reduce simulation efficiency. To address this issue, a radiator structure equivalence method based on wall heat transfer coefficient correction is proposed. First, the boundary inconsistency problem caused by geometric simplification in thermal simulations is analyzed, and a correction mechanism for the heat transfer coefficient is derived based on the principle of total heat flux conservation. Then, an equivalent heat transfer boundary model is constructed by reducing the number of fins and adjusting the convective heat transfer coefficient on the air side, thereby achieving functional equivalence of the thermal boundary. Finally, the proposed method is validated through simulations under various operating conditions, both at the radiator component level and the full transformer system level. The results demonstrate that the proposed approach significantly improves simulation efficiency while maintaining thermal accuracy, providing a practical solution for structural simplification and boundary condition consistency in transformer thermal modeling.